The present invention relates to a system and method for determining the mineral components in sheet material, and more particularly, for measuring the concentrations of individual components of mineral additives in a paper web in an on-line paper manufacturing process using a single x-ray source and multiple x-ray detectors, each detector with a different filter.
In addition to paper pulp fibers, paper manufactures use varying amounts of mineral additives, commonly referred to as xe2x80x9cashxe2x80x9d, to add strength, color and to change certain qualities like printability, opacity and brightness of their paper products. Such mineral additives include clay, kaolin, gypsum, calcium carbonate, barium sulfate, talc and titanium dioxide. As mineral additives are often cheaper than paper pulp, these inorganic materials are also used to lower manufacturing costs.
In present day paper manufacturing, clay, calcium carbonate and talc are commonly used mineral additives. While mineral additives have the above noted benefits, if too much mineral additive is used, i.e., if either the overall additive content exceeds an acceptable level or if any one of the components of these inorganic materials exceeds acceptable individual limits, the over-all quality and characteristics of the produced paper will be impaired. Additive limitations are particularly important to producers of cigarette paper who often use large quantities of titanium dioxide and calcium carbonate and have strict quality standards for their paper. Therefore, during the manufacturing process it is important to monitor and regulate both the total mineral additive content and the individual component concentrations of the mineral additives.
In the prior art, total mineral content has been determined by summing the measured individual component concentrations of the mineral additives present in the paper near the end of the paper making process. Either x-ray absorption or x-ray fluorescence have been employed to measure the individual component concentrations of the mineral additives. One or more measurement beams are directed through the continuously advancing paper web with each beam being received by a corresponding x-ray detector on the other side of the web. A computer, which receives signals representative of both the beam(s) and the detector(s), compares the signals and, using well-known or readily derived equations, computes the individual component concentrations of the mineral additives in the paper web.
Generally speaking, the gamma induced x-ray fluorescence approach for measuring total mineral content in paper is hampered by the following factors: low signal levels when counting photons; a slower count rate than the x-ray absorption method; it uses a complex temperature sensitive detector; and, it is difficult to calibrate for a non-homogenous sample, e.g., paper coated with inorganic materials such as clay, calcium carbonate, or titanium dioxide. For these reasons, no further discussion is provided on the x-ray fluorescence approach.
One particular problem associated with prior art x-ray absorption measurement systems having both a single x-ray source and detector has been measuring the total mineral content in a paper web having three mineral additives such as clay, calcium carbonate and titanium dioxide. While much success has been achieved measuring total mineral content in a paper web having any two-component mineral additive mixture of clay, calcium carbonate, and titanium dioxide with a single x-ray source instrument, if a third mineral additive is used, the accuracy of measuring the total mineral content of the paper web with such a device is compromised.
One solution to the above mentioned problem is to employ a single x-ray beam with a stacked ion chamber. With a stacked ion chamber, three ion chambers are positioned one on top of the other with each chamber having a different sensitivity to the detectable mineral additives. Another solution is to employ multiple x-ray beam sources operating at different energy levels centered near detection levels for the individual mineral additives.
With regard to the later solution, the use of multiple x-ray beam sources with corresponding detectors tuned to different sensitivities of the various mineral additives is very expensive. Further, the derived outputs using multiple x-ray beam sources are subject to reduced accuracy since the variance of each source must be accounted for in the measurements. The primary source of uncertainty, or instrument variance, is in the x-ray tube and associated high voltage power supply (HVPS), as it remains exceedingly difficult to construct a stable x-ray source.
As to the former solution, using a stacked ion chamber avoids the problem from root mean square (RMS) addition of multiple sources. However, a stacked ion chamber suffers from both the difficulty of positioning the chambers to achieve maximum signal collection, due to the typical bulky size of the chambers, and the poor signal collection in the chamber furthest from the single x-ray source.
Accordingly, the current focus in the art has been to attempt to solve the above-mentioned problems. One such prior art system is described in U.S. Pat. No. 5,854,821 (the ""821 patent) to Chase et al., which shows an improved x-ray measuring process for measuring mineral components, referred to as xe2x80x9cashxe2x80x9d, in paper having at least three such mineral components. To measure ash, the ""821 patent utilizes two adjacent x-ray sources placed on one side of a paper web, and two corresponding adjacent detectors placed on the opposite side of the paper web with the first source operating at an energy level higher than the second source. It is asserted that improved composition detection for ash in paper can be achieved in this manner. However, one skilled in the art will notice that operating such a system is still subject to reduced accuracy since variance of each x-ray source must also be considered. Further, as mentioned above, the use of multiple x-ray sources is a very expensive solution to the problem.
Therefore, there is a need for a cost effective method and system to measure individual component concentrations of at least three mineral additives in a paper web in an on-line paper manufacturing process that minimizes signal attenuation.
This need is currently met by the system and method of the present application wherein a single x-ray source is utilized with a multiple filtered detection arrangement to accurately determine the total mineral content in sheet material having at least three mineral additive components and also the individual concentrations of the mineral additive components.
In accordance with one aspect of the present invention, a system is provided for determining on-line measurements of various mineral additive components, such as clay, calcium carbonate, and titanium dioxide, in a paper web or paper sheets utilizing a single x-ray source and three filtered radiation detectors. Thin filters are used to shape and/or xe2x80x9chardenxe2x80x9d the detected spectrum of x-rays received by each detector. Hardening refers to a specific kind of change to the radiation spectrum where only low energies are reduced or eliminated. It is to be appreciated that the type and thickness of these filters are selected to maximize sensitivity differences for the various mineral components desired to be measured by the present system.
In a preferred application of the present invention, a single x-ray source generates an x-ray beam that passes through a continuously flowing paper web in a paper manufacturing process to at least three filters on the opposite side of the web. The beam, after passing through the filters, is detected by a corresponding number of detectors. The filters cause the measurement sensitivities of the detectors to vary and hence be tuned for particular mineral additives. Accordingly, each detector generates an analog signal in response to the detected energy level of the beam. It is to be appreciated that these detectors can consist of detectors known in the art or any combination of such known detectors, for example and not limited to, solid-state detectors, ion chamber detectors, and scintillation detectors.
Due to size and cost considerations, however, solid-state detectors are preferred, and more specifically, PIN (Positive-lntrinsic-Negative) diode detectors. In the present invention, although PIN diode detectors are normally operated in the conductive (or reverse biased) mode it is preferred that the PIN diode detectors be operated in a voltaic mode. Operating the PIN diode detectors in the voltaic mode ensures that no biasing of the diode detectors is required.
In response to the analog signals generated by the detectors and additional signals from conventional basis weight and moisture sensing instruments, a processor using one of a number of know computational methods derives measurements for total mineral content and also the individual component concentrations of titanium dioxide, calcium carbonate, and day in the paper web. One known method which can be used to derive such measurements is to calibrate each detector using known concentrations of mineral additives in samples to determine base curve coefficient constants. Using the determined base curve coefficient constants together with the noted analog signals, the processor solves a multiple linear regression to determine separate measurement solutions for each mineral component. Total mineral content can then be computed as the sum of the solutions of each individual component.
In accordance with another aspect of the present invention, a method is provided for determining on-line measurements of clay, calcium carbonate, and titanium dioxide in a paper web using a filtered radiation source with multiple detectors. The method comprises the steps of determining the various mineral component concentrations in paper by first directing a broadband radiation beam emitted from a radiation source towards the paper. After passing through the paper, portions of the radiation beam are modified by three filters to define a shaped radiation spectrum that maximizes the sensitivity differences for the desired detectable mineral components. A detector is associated with each filter such that the shaped radiation spectrum for each filter falls on the corresponding detector. Signals generated by the detectors in response to the shaped radiation spectrum are sent to a processor appropriately programmed to determine the individual component concentrations of the mineral additives in the paper web.
Further features and advantages of the invention can be determined by reference to the specification and drawings which are by way of example only and not in limitation of the invention which is defined by the claims and equivalents thereto.